This invention relates to microelectronic devices, and, more particularly, to a microelectronic device that is moved from one support to another support during fabrication.
Microelectronic devices are normally prepared by a series of steps such as patterning, deposition, implantation, growth, and etching that build up an electronic circuit on or near the top surface of a thin substrate wafer. Interconnection pads are placed on the surface of the wafer to provide connections to external leads or to other microelectronic devices. Such a microelectronic device is considered a two-dimensional structure in the plane of the substrate wafer. There are usually multiple layers of deposited conductors and insulators, but each layer is quite thin. Any height of the device in the third dimension perpendicular to the substrate surface is much less than the dimensions in the plane of the substrate wafer, and is often no more than a few thousand Angstroms.
The microelectronic devices or arrays of such devices are usually placed inside a protective housing called a package, with leads or connection pads extending out of the package. When the microelectronic devices are used, a number of the packages with their contained microelectronic devices are normally affixed to a base such as a phenolic plastic board. Wires are run between the various devices to interconnect them. There may be metallic traces imprinted onto the base to provide common power, ground, and bus connections, and the base itself has external connections. Such boards with a number of interconnected devices are commonly found inside both consumer and military electronics equipment. For example, an entire microcomputer may be assembled as a number of microelectronic devices such as a processor, memory, and peripheral device controllers mounted onto a single board.
The present inventors have determined that for some applications it would be desirable to stack and interconnect a number of such two-dimensional microelectronic devices, fabricated on a substrate wafer, one on top of the other to form a three-dimensional device. The stack might also include other circuit elements such as interconnect layers and thin film sensors as well. To interconnect the stacked wafers using leads that extend from the pads on the top of one wafer to the pads on the top of another wafer, around the sides of the wafers, or using plug interconnects or the like, would be clumsy, space consuming, and impossible to do for the case of highly complex circuitry requiring many interconnects.
In considering fabrication techniques to produce such three-dimensional, stacked devices, the fragility of the devices is a concern. The individual substrate wafers and their microelectronic circuitry are usually made of fragile semiconductor materials, chosen for their electronic characteristics rather than their strength or fracture resistance. The selected fabrication technique cannot damage the circuitry that has already been placed onto the substrate wafer.
Thus, there is a need for a method to fabricate three-dimensional microelectronic devices using stacked substrate wafers with circuitry already on them. The present invention fulfills this need, and further provides related advantages.